Seismic Design of Foundations for Steel-Framed Buildings: A Canadian Perspective

Provisions for the seismic design of foundations have changed significantly in recent editions of Canadian codes and standards. Considering that the foundations are constructed using reinforced concrete, these requirements were developed mainly based on Canadian studies of the seismic response of RC shear walls. Hence, they may be less adapted to steel braced frame systems and the particularities of their seismic behaviour and design, such as a distributed yielding mechanism and a differentiation between the nominal and probable capacities of ductile elements. This may lead to overly conservative design estimates of the demands on foundations and the total building drifts, adversely affecting not only the foundations themselves by increasing their size and cost, but also the choice of steel as the material for the seismic force resisting system.This paper presents an overview of Canadian provisions for the seismic design of foundations and critically assesses their applicability to the foundations of steel concentrically braced frames. The design procedure is illustrated with the example of 3-storey steel buildings with X-type tension-compression bracing, located in Vancouver and Montreal and designed following the National Building Code of Canada and the associated editions of the concrete and steel design standards. The seismic response is examined using nonlinear time history analysis, where a numerical model developed in OpenSees integrates inelastic frame behaviour and nonlinear soil response. The results clearly demonstrate the shortcomings of current foundation design procedures when applied to steel frames. Possible solutions to overcome these limitations with an improved design methodology are discussed.